A critical requirement of advanced semiconductor processing and a key measure of process capability is a tight uniformity of the process parameters, such as uniformity of the smallest feature dimensions in a circuit, or the smallest space, commonly referred to as the Critical Dimension (CD); or the uniformity of alignment between different patterning layers in a circuit, commonly referred to as Overlay (OL) over the entire wafer area. This requirement becomes increasingly challenging given the continued reduction of feature sizes required through scaling (the industry claims to be at the 10 nm Technology Node now) and the increasing size of wafers (currently at 300 mm diameter with 450 mm diameter expected in the near future). For example, if the critical dimension (CD), which is indicative of the minimum feature size of a pattern, is only a few tens of nanometers, and the requirement is for a <5% variation of CD, then the process control must deliver CD which varies <1 nm. The control of any process to that degree of accuracy is very challenging. For this reason, a secondary adjustment of the results of a primary process (in this case, CD) is desirable. This means that the primary process (for example, a photolithographic exposure process or a substrate etch process) is performed and the result obtained from that process does not satisfy the final process control requirement, thus requiring a secondary adjustment to the primary process.
For example, a photolithographic exposure process may be performed using an optical transfer of a circuit pattern from a photomask onto a silicon wafer using a projection lithography exposure. In semiconductor patterning processes, prior to exposure, substrates are typically coated with a photosensitive polymer (photoresist) which is altered by exposure to radiation as provided by the projected image of a photomask. The projected energy pattern, as absorbed into the photoresist, alters the film material so that it can be selectively removed in a subsequent chemical development process. The developed resist can then, for example, be used as a mask during a subsequent etch process to transfer the photoresist pattern into the underlying substrate for the purpose of generating circuit patterns. The photomask pattern, and the characteristics of the photolithographic projection system, are carefully designed to deliver an image with controlled CD to each location on the wafer. However, many process variations, such as projection illumination uniformity in the image projection system, or photoresist thickness variations during coating processes, may cause deviations of the CD from target values. These deviations can degrade circuit performance and can be difficult to control.
The CD uniformity of patterns on semiconductor substrates can be obtained using CD mapping tools provided by several vendors such as KLA-Tencor (Optical CD metrology) or Applied Materials (SEM CD metrology). These tools are capable of measuring circuit pattern CD at many points on the substrate, thereby generating a 2D map of CD errors across the substrate. In many cases, measured CD maps display a great deal of repeatability from one substrate to the next. Repeatable patterns are signatures of particular processes or process tools which were used to generate the pattern. Such repeatable patterns are referred to as substrate signatures. A substrate signature is mostly determined by the specific semiconductor processing equipment used to process the wafer. Thus, if the equipment used for substrate processing is altered or replaced, the substrate signature may also be altered and may need to be re-measured.
The substrate signature is typically represented by a 2D map of CD errors as measured in nanometer units. This is called a substrate signature map. This map can be used for the purpose of CD error compensation through a secondary adjustment of the primary (exposure) process by location dependent pre-exposure or post-exposure dosing adjustment. For the case of secondary adjustment of the exposure process, the amount of pre- or post-exposure correction required for each position in the substrate signature map may be measured and stored, forming a predetermined 2D correction map which can be applied to subsequent substrates for the purpose of correction. For the case of secondary adjustment of the exposure process, the substrate signature map, typically representing CD error as a function of position in nanometer units, may need to be processed into a substrate correction map, which typically represents deposited energy per pixel, for the purpose of exposing a photoresist film, as a function of pixel position (e.g., in units of milli-Joules), or alternatively, deposited energy per area as a function of pixel position (e.g., in units of milli-Joule/cm2). In this example, the process of transforming a substrate signature map into a substrate correction map requires knowledge of photoresist photochemical exposure parameters which are typically available from photoresist vendors.
Typically, the substrate signature map is a substantially repeatable map of process control variations characteristic of particular pieces of equipment or individual process steps applied to semiconductor substrates. Other processing steps, such as the application of photopolymers or deposition and etching of thin films on the substrate, may also exhibit substrate signatures which can be measured and compensated through secondary adjustment. For the case that a substrate signature is caused by thermal characteristics of a process step, for example a baking or etching step, which cause undesirable process variations, for example CD variations, the thermal or energy signature of the process may be measured and applied to the substrate for the purpose of secondary adjustment. For the case of secondary adjustment of a thermal or energy process, the substrate signature map may be measured and stored, forming a predetermined thermal input signature which can be applied to subsequent substrates for the purpose of correction. For the case that a substrate signature is caused by thermal characteristics of a process step, the substrate signature map, typically representing CD error as a function of position in nanometer units, may need to be processed into a substrate correction map, which typically represents deposited energy per pixel into a substrate, for the purpose of altering local temperature, as a function of pixel position (e.g., in units of milli-Joules), or alternatively, deposited energy per area as a function of pixel position (e.g., in units of milli-Joule/cm2). In this example, the process of transforming a substrate signature map into a substrate correction map would require knowledge of the local temperature rise that is necessary to effect a particular CD target.
U.S. Patent Publication Nos. 2015/0147827 A1, 2015/0146178 A1, 2015/0212421 A1, and 2016/0048080 A1, each to deVilliers et al. (henceforth collectively referred to as “deVilliers”) address such a secondary adjustment for a number of processes, including CD-uniformity. DeVilliers describes achieving the secondary adjustment by projecting a modulated intensity map onto the wafer using a spatially modulated array, such as a commercially available micro-mirror micro-electromechanical systems (MEMS) device. Such micro mirror arrays (MMA) are frequently called “Digital Light Processors (DLP)” or “Grating Light Valves (GLV)” and are commonly used in digital motion picture projectors. Since the images so projected are in the visible light spectrum, commercially available MMAs are optimized for the visible band of electromagnetic radiation (between 400 nm and 800 nm). The majority of the applications in need of adjustment in semiconductor processing require shorter wavelengths. Wavelengths of 193 nm and 248 nm are typical. However, light at this shorter wavelength is more energetic than visible light and is capable of damaging certain MMA devices.
FIG. 1 depicts a projection imaging system for a digital light processor of the type described in deVilliers. Light from a radiation source 101, for example a light emitting diode device (LED) emitting UV radiation, is collimated by lens 102 into beam 103 which substantially uniformly illuminates MMA device 104. MMA device 104 is driven with the desired patterns by an electronic system (not shown). Lens 107 forms an image of said MMA pattern onto substrate 109 which is chucked to substrate holder 111. In the figure, the MMA pattern is shown to generate an image beam 106 from exemplar point 105 on MMA device 104, which is then projected onto corresponding point 110 on substrate 109 via projection beam 108 by means of projection lens 107. In turn, many other exemplar points on MMA device 104 will be found to project to corresponding points on substrate 109, in a manner consistent with well-known imaging optics. With this method, the entire pattern generated by MMA device 104 is projected simultaneously to a corresponding image on substrate 109. Depending on the focal length of lens 107, the distance of lens 107 to MMA device 104, and the distance of lens 107 to substrate 109, the image on substrate 109 may be reduced or magnified in size with respect to the image on MMA device 104, following well-known principles of lens theory.
The general use of scanning light beams to process substrates, for example using a beam generated by a laser, is known in the art. Many methods of using scanned laser beams for surface patterning or modification have been described in the art. A typical application modifies surfaces or surface films by melting or ablating the material, for example, to provide annealing or to achieve small surface relief features.
One of the earliest commercial applications of lasers was a production laser cutting machine used to drill holes in diamond dies, made by the Western Electric Engineering Research Center (1965). In 1967, the British pioneered laser-assisted oxygen jet cutting for metals. In the early 1970s, laser technology was put into production to cut titanium for aerospace applications. There are many different methods for cutting using lasers, with different types used to cut different materials. Some of the methods are vaporization, melt and blow; melt, blow and burn; thermal stress cracking; scribing; cold cutting; and burning stabilized laser cutting. For a summary of these methods, see https://en.wikipedia.org/wiki/Laser_cutting.
In semiconductor manufacturing, commercial laser processing tools (for example, the Ultratech LSA100A) provide solutions to the difficult challenge of fabricating ultra-shallow junction and highly activated source/drain contacts. Laser Spike Annealing (LSA) operates at near-instantaneous timeframes (micro-seconds) at temperatures up to 1,350° C. At these temperatures, nearly full activation of dopants with minimal diffusion is achieved in micro-seconds timeframes. The laser beams can also provide chemical modification or induce chemical removal or deposition to occur on the substrate or to thin films applied thereon. The ability of lasers to accurately deliver large amounts of energy into confined regions of a material allow the modification of surface chemistry, crystal structure, and/or multiscale morphology without altering the bulk. A summary of such applications is contained in Chapter 13 of Laser Precision Microfabrication; Editors: Koji Sugioka, Michel Meunier and Alberto Piqué; ISBN: 978-3-642-10522-7 (Print), 978-3-642-10523-4 (Online), 2010.
A process called scanning photolithography, for example, alters the surface or chemistry of a surface film on a substrate so that a subsequent step may remove or otherwise reveal surface topography or features. For example, scanning photolithography is used in the production of photomasks where it generates arbitrary patterns for subsequent replication onto substrates using the well-known processes of contact or projection photolithography. The Applied Materials ALTA® 4700 Mask Pattern Generation system, introduced in 2004, provided the industry with mask layers for 90 nm and most 65 nm critical levels. This system, the laser-based DUV ALTA 4700, featured a 42×, 0.9 NA objective lens, providing superior mask resolution, pattern fidelity, critical dimension control and placement performance. See http://www.appliedmaterials.com/company/news/press-releases/2004/11/applied-materials-new-alta-4700-laser-mask-writer-takes-on-65 nm-critical-layer-manufacturing. The Applied Materials technology is described in, for example, U.S. Pat. Nos. 5,386,221 and 7,483,196B2.
Alternatively, Swedish company Mycronic, AB is applying scanning photolithography to display technology and to advanced electronic packaging applications. This technology is described in, for example, U.S. Pat. No. 8,822,879B2 and in U.S. Patent Application Publication No. US20150125077A1.
Prior to applying scanning photolithography to display technology and advanced electronic packaging applications, Mycronic (under the name of Micronic) applied scanning photolithography to the manufacturing of mask layers. This technology is described in, for example, U.S. Pat. Nos. 6,624,878B1; 7,446,857B2 and 8,958,052B2.
During scanning photolithography, a focused beam of UV light is scanned and directed onto a thin film, called a photoresist or resist, which is then modified by the light (or exposed) resulting in high resolution micro- or nano-scale patterns. Here, the UV light chemically modifies the film by breaking or cross linking bonds in resist molecules. During a subsequent chemical development process, the exposed regions of the film, for the case of a positive photoresist, are chemically removed, whilst the regions of the photoresist film not exposed remain. In the case of a negative photoresist, the regions which received no exposure are chemically removed and regions which receive exposure remain. An example of such a beam scanning tool for the purpose of exposing substrate films with arbitrary patterns using UV light beams is the ALTA photomask patterning tool manufactured by Applied Materials, Inc. That tool uses a spinning polygonal mirror wheel to scan beams across a substrate.
Such beam scanning schemes typically seek to produce an array of patterns on a substrate, with said patterns comprised of features in various sizes ranging from small to large. There is a strong desire to make the minimum features as small as possible. In prior art scanning, the minimum feature size is typically similar, or slightly larger, than the size of the beam. For laser beam surface patterning, a desired minimum surface feature may be broken into a small number of pixels (between four and nine for example). Typically in this case the minimum size feature that is being patterned by the beam is the same size or larger than the beam spot size on the substrate. For example, a scanning beam spot of 1 micrometer diameter can be used to produce arbitrary patterns on a substrate, said patterns placed at arbitrary locations, with a minimum feature size of approximately 3 micrometers.
Energy input into a substrate or substrate film by a beam or image is generally referred to as “dose,” which is typically indicated or measured in units of energy per square area, for example, mJ/cm2.
In general, there are two main types of beam scanning techniques: vector scanning and raster scanning. During vector scanning, patterns on the substrate are generated by using a beam steering method that moves the beam on the substrate from one arbitrary point A to a second arbitrary point B. During scanning, the beam may be “blanked”—or rapidly turned on and off—before, during, or after the motion from point A to B, in order to achieve a desired energy input pattern into the substrate. In the vector scanning method, after a path from point A to point B is defined, subsequent scanning defines a path from point C to point D, E to F, etc., in rapid succession until the entire desired pattern is written.
During raster scanning, on the other hand, the beam is swept rapidly back and forth across the substrate in one direction, whilst being swept slowly across the substrate in an orthogonal direction, so that after some time the beam has traversed all points on the substrate. During this scanning process, the beam is rapidly blanked (i.e., turned and off) under electronic control so that the desired pattern is transferred to the substrate.
Some writing strategies combine both, vector and raster scanning (see for example U.S. Patent Application Publication No. US20070075275).
In general, for both scanning schemes, it is desired that the beam on/off switching time (i.e., blanking time) be sufficiently short during laser beam writing, such that the scanning spot will impart a sufficiently small minimum region of the substrate with energy input. Thus, if the blanking time is sufficiently short, the size of the resulting minimum features will be comparable to the size of the beam. We refer to the minimum imparted energy spot width on the substrate resulting from said fast blanking step as the down-track pixel size. It is desired that the down-track pixel size and the beam diameter be comparable in size in order to achieve the optimal minimum possible feature size on the substrate. In this case the minimum feature will resemble a dot or disk. Said minimum feature size can be measured using a 1D or 2D criteria (for example, using a full-width-at-half-max criteria). The minimum feature size is typically referred to as the critical dimension (CD).